Conventionally, wirings have been formed in microscopic wiring grooves, holes or resist opening portions which are provided on the surfaces of substrates of semiconductor wafers and the like, bumps (protruded electrodes) to be electrically connected to electrodes and the like of packages have been formed on the surfaces of substrates. As the methods for forming the wirings and bumps, an electrolytic plating method, a vacuum evaporation method, a printing method, a ball bump method and the like are known, for example. With increase in the number of I/O, and narrowing of pitches in semiconductor chips, an electrolytic plating method which enables miniaturization and is relatively stable in performance has come to be used frequently.
When wiring or a bump is formed by an electrolytic plating method, a seed layer (a power feeding layer) with low electric resistance is formed on the surface of a barrier metal which is provided in a wiring groove, a hole or a resist opening portion on a substrate. On the surface of the seed layer, a plated layer, i.e. a plating film grows.
In general, a substrate which is plated has electric contacts in a peripheral portion thereof. Consequently, in a central portion of the substrate, a current which corresponds to a combined resistance of an electric resistance value of a plating solution and an electric resistance value of a seed layer from the central portion of the substrate to the electric contracts flows. Meanwhile, in the peripheral portion (the vicinity of the electric contacts) of the substrate, a current which substantially corresponds to the electric resistance value of the plating solution flows. That is, a current is difficult to flow correspondingly to the electric resistance value of the seed layer from the central portion of the substrate to the electric contact points. The phenomenon in which a current concentrates on the peripheral portion of the substrate is called a terminal effect.
As the shape of the substrate which is plated by an electrolytic plating method, a circular substrate and a quadrangular substrate are known (refer to Japanese Patent Laid-Open No. 09-125294 and Japanese Examined Patent Publication No. 03-029876, for example).
In a circular substrate, a distance from a central portion of the circular substrate to a peripheral portion of the substrate is the same throughout an entire circumference of the substrate. Consequently, the terminal effect at the time of plating the circular substrate occurs substantially similarly throughout the entire circumference of the substrate. Accordingly, when plating is applied to the circular substrate, a plating speed in the central portion of the substrate is lower as compared with that in the peripheral portion of the substrate, and the film thickness or thickness of the layer of the plating film in the central portion of the substrate becomes thinner than that of the plating film in the peripheral portion of the substrate. Conventionally, in order to restrain reduction of in-plane uniformity of the film thickness by a terminal effect, an electric field which is applied to the circular substrate has been regulated by using a regulation plate as is disclosed in Japanese Patent Laid-Open No. 2005-029863, for example, while a current is uniformly supplied from the peripheral portion of the circular substrate.
However, in a polygonal substrate, the distance from a central portion of the polygonal substrate to a peripheral portion of the polygonal substrate differs depending on the position. That is, the distance from the central portion of the substrate to a central portion (a central portion between vertexes) of a side of the polygonal substrate is relatively short, and a distance from the central portion of the substrate from a vicinity of the vertex of the polygonal substrate is relatively long. Consequently, the terminal effect at the time of plating the polygonal substrate occurs ununiformly throughout the entire perimeter of the substrate.
In studying the plating method and the plating apparatus for a polygonal substrate, the present inventors have studied the change of a current density distribution on the polygonal substrate accompanying advance of electrolytic plating. FIG. 21A to FIG. 21D are schematic views showing a change of the current density distribution accompanying advance of plating when plating is applied to a quadrangular substrate which is an example of a polygonal substrate. FIG. 21A shows a current density distribution of the quadrangular substrate at an initial stage of plating. As shown in FIG. 21A, in a center portion C1 of a quadrangular substrate S1, an electric resistance value is higher as compared with that in a peripheral portion due to a terminal effect, and therefore, a current density is the smallest. In the stage shown in FIG. 21A, a side central region A1 of the quadrangular substrate S1 is at a relatively shorter distance from the center portion C1 of the quadrangular substrate S1, and therefore, an electric resistance value in the side central region A1 is lower as compared with an electric resistance value in a corner region A2. Consequently, an electric field relatively concentrates on the side central region A1, and a current density becomes high. Meanwhile, at the corner region A2 in a vicinity of a vertex of the quadrangular substrate S1 and an intermediate region A3 which is located between the side central region A1 and the corner region A2, distances from the center portion C1 are longer as compared with the side central region A1, and therefore, electric resistance values are higher as compared with that in the side central region A1. Consequently, in the stage shown in FIG. 21A, an electric field does not concentrate on the corner region A2 and the intermediate region A3 so much as on the side central region A1.
When plating advances, an electric field also starts to concentrate on the corner portion A2 which is at a relatively long distance from the center portion C1, as shown in FIG. 21B. Meanwhile, current densities in the side central region A1 and the intermediate region A3 become smaller as compared with current densities in the side central region A1 and the intermediate region A3 shown in FIG. 21A. This is because as plating advances and the film thickness becomes larger, the influence of the terminal effect gradually becomes smaller.
Plating further advances, and the film thickness becomes larger, whereby concentration of the electric field onto the side central region A1 is relieved, as shown in FIG. 21C. Consequently, a difference between the current density of the side central region A1 and the current density of the intermediate region A3 becomes smaller as compared with FIG. 21A and FIG. 21B. Meanwhile, concentration of the electric field onto the corner region A2 does not change so much even when plating advances.
When plating further advances, the difference of the current density in the side central region A1 and the current density in the intermediate region A3 substantially disappears, as shown in FIG. 21D. Meanwhile, in the corner region A2, concentration of the electric field continuously occurs.
As explaining in relation to FIG. 21A to FIG. 21D, in the side central region A1 of the quadrangular substrate S1, the electric field concentrates at the initial stage of plating, and the current density becomes smaller as plating advances. In contrast to this, in the intermediate region A3 of the quadrangular substrate S1, the current density is smaller than the current density in the side central region A1 at the initial stage of plating, and as plating advances, the difference from the side central region A1 becomes smaller. Accordingly, in the side central region A1, a film thickness of plating tends to be larger than the film thickness of plating in the intermediate region A3.
Meanwhile, although in the corner region A2 of the quadrangular substrate S1, concentration of the electric field is somewhat small at the initial stage of plating, the electric field continues to concentrate consistently from the initial stage of plating to an end time. Consequently, in the corner region A2, the film thickness tends to be larger as compared with the film thickness in the side central region A1 and the intermediate region A3.
As above, the terminal effect in the polygonal substrate differs depending on the position of the peripheral portion of the substrate, and therefore, ununiformity in the current density distribution occurs in the peripheral portion of the substrate. Consequently, the present inventors have found out that in-plane uniformity of the film which is plated on the polygonal substrate is lower as compared with the in-plane uniformity of the film plated on a circular substrate.
The present invention is made in the light of the above described problem, and an object of the present invention is to enhance in-plane uniformity of a film which is plated on a polygonal substrate.